1. Field of the Invention
The present invention relates generally to the use of magnetic fields, and more particularly, to methods of using magnetic fields to uniformly induce electric fields for therapeutic purposes.
2. Related Art
Exposure to electromagnetic fields (EMFs) has become an increasingly useful tool in the treatment of many medical conditions. For example, exposure to time-varying magnetic fields is an accepted method of accelerating bone and wound healing. For example, EMFs may be used to limit damage to a heart during a heart attack and to protect bone marrow during chemotherapy and x-ray therapy for destruction of tumors.
When an EMF is applied to a cell, the electric field acting on the cell is the main mechanism by which the EMF affects the cell. For most purposes, the use of a low frequency time-varying magnetic field is the most convenient and controllable method of causing an electric field to appear across the tissue to be treated. A time-varying magnetic field may be created external to the body (for example with a pair of coils and a time-varying current source). When this field enters a body, it induces (by Faraday's Law) a time-varying electric field. It is fairly straightforward to create a uniform magnetic field in a body because the body's magnetic properties are quite uniform. However, the induced electric field is very non-uniform because the body's electrical conductivity may vary enormously from organ to organ (e.g., lung to heart) and within an organ (e.g., heart muscle to heart blood).
This lack of uniformity represents a serious limitation in the therapeutic application of time-varying magnetic fields. A good example of this limitation is in the use of magnetic fields to limit damage to the heart after an ischemic event (e.g., heart attack). Application of the magnetic field for a period of 30 minutes or more induces activation of heat shock proteins (hsps) in the cells of the heart muscle. These hsps act to protect the heart from cell death (necrosis) during the period in which the stoppage of blood flow (ischemia) causes cell stress. The problem that exists with this technique is that the induced electric fields vary so greatly that in many regions of the heart the induced electric field is not great enough to cause the cells to produce hsps. For example, the lung is a high resistance region adjacent to the heart. As a result, if the induced electric field passes through both the lung and heart, most of the field will appear across the lung and very little in the heart. Even if the induced electric field is applied in a direction that does not cross the lung, there will be regions in the heart that do not experience a significant electric field because the blood has such a low conductivity relative to the heart muscle.
Which regions of an organ do not experience a significant electric field depends critically upon the direction of the applied magnetic field, and thus the direction of the induced EMF. One proposed solution may be to simply apply fields in the x, y and z directions simultaneously. This however does not work since the vector sum of these fields would be simply a new magnetic field in a single direction.